Pseudomonas aeruginosa is one of the chief causes of infection in patients with acute leukemia and other severe malignancies whose white blood cell counts are less than or equal to 1000/ml. The prime objective of this study has been to monitor the epidemiology of P. aeruginosa in cancer patients and to study its host-parasite interaction and susceptibility to antibiotics. Recovery rates are lower than in previous years. Serotyping of the 66 P. aeruginosa strains recovered from 21 patients revealed that only two patients had multiple serotypes (2) present. No transmission of P. aeruginosa between patients could be detected. The identification schema developed in our laboratory to speciate pseudomonas strains other than P. aeruginosa continues to be successfully used. A schema for identification of less common non-sacchardytic Pseudomonas and Alcaligenes spp. has now been successfully evaluated with 200 strains of these species. Studies of non-agglutinating pseudomonas antibodies have been temporarily postponed.